(2)  
The magnetic heading is recorded by the flight recorder system (31-31-00).

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 F.  The MHRS requires 115 volts (400 Hz) for primary power, 5 volts ac for panel illumination, 26 volts ac for synchro transmitter excitation, and 28 volts dc for relay control. Primary power for the captain's system is supplied from the standby bus and primary power for the first officer's system is supplied from flight instrument bus No. 2.
 2.  Magnetic Field Sensor
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 A.  The magnetic field sensors are mounted on prealigned brackets in the wing tips to minimize magnetic disturbances from the surrounding airplane structure. Each sensor has a preindexed mounting plate permitting the sensor to be removed and replaced on the bracket without system recalibration.
 B.  The magnetic field sensor consists of a sensing device mounted on a pendulum. The pendulum is suspended in a damping fluid so that it will always seek a vertical position and is free to move approximately 30 degrees in its pitch and roll axes. The sensing device on the pendulum lies parallel to the horizontal component of the earth's magnetic field, permitting the horizontal component to be sensed while rejecting the vertical component. This magnetic sensing is accomplished by driving the magnetic core of the three sensing coils into saturation to degauss the core, and then decreasing the driving flux to allow the earth's magnetic field to re-enter the core. The three sensor windings on the core provide a three-wire output voltage proportional to the earth's magnetic field strength and magnetic field direction.
 3.  Magnetic Compensator
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 A.  The magnetic compensator is a remote unit used to correct the magnetic signal for single-cycle magnetic error induced in the compass system by magnetized parts in the airplane structure or by dc currents in the airplane wiring.
 B.  The compensator provides east-west (E-W) and north-south (N-S) adjustments to compensate for the single-cycle error. Compensation is achieved by multiturn precision potentiometers connected across a regulated power supply to provide small dc currents through the magnetic field sensor secondary coils. The combined effect of these currents is sufficient to correct for 8 degrees of single-cycle error.
 C.  Three test points (N-S compensation, common, and E-W compensation) are provided on the compensator panel to permit measurement of the DC voltages across the E-W and N-S potentiometers. The measured voltages are recorded on placards affixed to the panel. If replacement of the compensator is required, the new compensator will provide the same compensation when its E-W and N-S potentiometers are adjusted to the original voltage values recorded on the placards attached to the face of the originally installed compensators. On airplanes with adjustment markers installed on brackets adjacent to magnetic compensators, the voltages are also recorded on these magnetic compensator adjustment markers.
 4.  Compass Controller
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 A.  The compass controller provides mode switching, set heading control, and SYNC annunciation for the MHRS. A toggle switch on the front panel selects either the DG or SLAVED mode of operation, a set heading knob allows the flight instrument compass cards to be manually set to a desired heading, and an annunciator is provided to indicate system synchronization.
 5.  Compass Coupler
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 A.  The compass coupler has circuits which process the long-term compensated magnetic field sensor signal with the short-term stable INS heading signal in order to produce accurate and stable magnetic heading outputs.
 B.  The compass coupler accepts 115-volt 400-Hz single phase power from the aircraft electrical system and in turn supplies power to the magnetic field sensor, magnetic compensator, and compass controller.
 C.  Seven heading output synchro transmitters are provided in each coupler. The transmitters are independent and isolated from each other. In addition, each coupler has a single heading hold synchro transmitter which provides clutch-coupled heading hold information to the autopilot.
 D.  The synchro transmitters may be driven at a fast rate to provide automatic synchronization. This rate is approximately 700 degrees per minute when fast syncing is required.
 E.  The compass coupler provides a signal to operate the synchronization annunciator on the compass controller. The signal voltage changes polarity to indicate the direction in which the system is out of synchronization.
　
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